Electrically evoked brainstem response measurements via implant prosthesis

ABSTRACT

A method of measuring electrically evoked auditory brainstem responses of a patient or animal body is provided. The method includes surgically implanting an auditory prosthesis having an electrode array, the electrode array positioned either intracochlear or substantially proximate a brainstem of the body. At least one electrode is stimulated in the electrode array. Electrically evoked auditory brainstem responses resulting from said stimulation are recorded using, at least in part, an electrode in the electrode array as a negative electrode, and a positive electrode positioned substantially proximate the vertex of the head of the body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/194,114, filed Jul. 29, 2011, which in turn claims priority from U.S.provisional application No. 61/368,892 filed Jul. 29, 2010. Each of theabove-described applications is hereby incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present invention relates to measurement of electrically evokedAuditory Brainstem Responses (eABR), and more particularly tomeasurement of eABR via an implant prosthesis.

BACKGROUND ART

The electrodes of commercially available cochlear implant or auditorybrainstem implant (ABI) systems may be used for acute electricalstimulation, obtaining important information about the electrodes itselfand/or the implant system, or recording electrically evoked potentials(EEP), such as the compound action potential (CAP). The development ofelectrically evoked CAP responses enabled measurement of CAP responsesvia a cochlear implant. However, recordings of objective measurements ofvarious other types of EEP via a cochlear or ABI implant are stillproblematic.

Evoked potentials include early, middle and late latency auditoryresponses, as described by Katz J., Handbook of Clinical Audiology,Williams & Wilkins 4^(th) Edition, 1994, which is hereby incorporatedherein by reference. By definition, the CAP is an alternating currentresponse which is generated by the cochlear end of the VIIIth CranialNerve, and it represents the summed response of the synchronous firingof thousands of auditory nerve fibers, as described by: Ferraro et al.,The Use of Electrocochleography in the Diagnosis, Assessment, andMonitoring of Endolymphatic Hydrops, Otolaryngologic Clinics of NorthAmerica; 16:1, pp. 69-82, February, 1983; and Hall J. W., Handbook ofAuditory Evoked Responses, Allyn and Bacon; Needham Heights, Mass.,1992, each of which is hereby incorporated herein by reference. Theauditory brainstem response (ABR) is an electrical signal evoked fromthe brainstem of a human or other mammal by the presentation of aspecific signal (Katz, 1994). The first recording of an ABR potential inman was performed by Jewett D. L. et al., Human Auditory EvokedPotentials: Possible Brain Stem Ccomponents Detected on the Scalp,Science 1970; 167: p. 1517-8, which is hereby incorporated herein byreference. The relationship of specific wave components of the ABR tothe components of the auditory pathway can be represented as follows:wave I: Cochlear Action Potential (CAP), distal CNVIII; wave II:proximal CNVIII; wave III: Cochlear Nuclei; wave IV: Superior OlivaryComplex; and wave V: Lateral Lemniscus. The definition of peaks isdescribed in Jewett D. L. et al., Auditory-evoked Far Fields Averagedfrom the Scalp of Humans, Brain. 1971; 94(4): p. 681-96, which is herebyincorporated herein by reference.

Electrically evoked Auditory Brainstem Responses (eABR) may be obtainedby recording a series of potentials with, in part, one or more scalpelectrodes. The response typically occurs within 10 msec after onset ofa pulsatile stimulus. The pulsatile stimulus may be provided, forexample, by an electrode associated with a cochlear implant or ABI.

In many instances, electrically evoked Auditory Brainstem Responses(eABR) can provide information useful for implants. For example, eABRcan provide important information regarding hearing, and using aspecific stimuli may provide electrode specific hearing information of asubject. While the CAP measurement can obtain information about nervefibres within the cochlea, eABR has the ability to check auditorypathway, i.e. assessing the functions of the inner ear, VIII cranialnerves, and various brain functions of the lower part of the auditorysystem.

For eABR via an ABI, the electrode array is placed directly on thecochlear nucleus, Therefore, the eABR recordings cannot includerecordings of waves I, II and partially wave III, and occurs 1-2 msearlier than it does with a cochlear implant. Since stimulatingartifacts are present substantially immediately after the presentedstimuli and at the beginning of the recordings, analysis of eABR via ABIis typically more difficult than it is for eABR via a cochlear implant.In the fitting process, for ABI subjects who never heard or have onlyvery little experiences with the hearing (for example, children), eABRbecomes a very important objective measurement. Compared to cochlearimplant patients, eABR may be of higher importance for ABI patients assome objective measurements often used in the fitting process incochlear implant patients, may not be elicitable, i.e. electricallyevoked stapedial reflex measurement (eSR).

Particular uses of eABR are listed below.

-   -   eABR may serve as an aid for the intra-operative or        post-operative confirmation of electrode placement, and        functionality of the implant (see: Behr R. et al., The High Rate        CIS Auditory Brainstem Implant for Restoration of Hearing in        NF-2 Patients, Skull. Base 17(2), 2007, p. 91-107; and Bahmer A.        et al., Recording of Electrically Evoked Auditory Brainstem        Responses (E-ABR) with an Integrated Stimulus Generator in        Matlab, Journal Neuroscience Methods 173(2), 2008, p. 306-314,        each of which is hereby incorporated herein by reference. eABR        has become a standard measurement for use in judging the proper        placement of the electrode paddle on a cochlear nucleus.    -   eABR may serve an aid for diagnostic assessment of a patient        (see Gibson et al., The Use of Intra-operative Electrical        Auditory Brainstem Responses to Predict the Speech Perception        Outcome after Cochlear Implantation, Cochlear. Implants Int., 10        Suppl 1 2009, p. 53-57, which is hereby incorporated herein by        reference). In particular, eABR can be a tool for the assessment        and monitoring of audiologic, otologic and neurologic disorders        (i.e. acoustic tumor monitoring).    -   eABR may be used as an aid for the post-operative programming of        a hearing implantable device, especially in difficult-to-fit        patients such as children (see: Brown C. J. et al., The        relationship between EAP and EABR thresholds and levels used to        program the nucleus 24 speech processor: data from adults, Ear        Hear. 21(2), 2000, and McMahon et al., 2008, p. 151-163; and        McMahon C. M. et al., Frequency-specific Electrocochleography        Indicates that Presynaptic and Postsynaptic Mechanisms of        Auditory Neuropathy Exist, Ear Hear. 29(3), 2008, p. 151-163,        each of which is hereby incorporated herein by reference).    -   eABR may provide additional information on psychoacoustic        thresholds for each intracochlear or electrode (see Brown et        al., 2000)    -   eABR may be used as an aid to provide quantitative information        on auditory pathway (See: Polak M. et al., Evaluation of Hearing        and Auditory Nerve Function by Combining ABR, DPOAE and eABR        Tests into a Single Recording Session, J. Neurosci. Methods        134(2), 2004, p. 141-149; and Gibson et al., 2009).

Obtaining eABRs can be cumbersome. An external system for recording istypically required, which then has to interface/synchronize with thestimulation system. Various recording electrodes need to be positionedon the patient, the location of which may be susceptible to movement bythe patient.

Furthermore, commercially available recording systems often recordstimuli artifacts together with the physiological response. Theseartifacts may be much higher than the physiological response (artifactsare up to several decade times higher than a physiological response).Thus, it is often very difficult to judge if the physiological responseis present. Sometimes, the artifacts are confused with the physiologicalresponse, and thus the final interpretation of the results may lead toan incorrect interpretation. For intraoperative measurement, whenrelying only on the eABR response, inappropriate judgement may have avery dramatic influence on postoperative performance with the implantsystem. For eABR recordings, only an alternating artifact cancellationmethod is being used commercially (for example, see Polak et al., 2004).However, this method is often not satisfactory in cancelling out thestimuli artifacts.

SUMMARY OF THE INVENTION

In accordance with a first embodiment of the invention, a method ofmeasuring electrically evoked auditory brainstem responses of a patientor animal body is provided. The method includes surgically implanting anauditory prosthesis having an electrode array, the electrode arraypositioned either intracochlear or substantially proximate a brainstemof the body. At least one electrode is stimulated in the electrodearray. Electrically evoked auditory brainstem responses resulting fromsaid stimulation are recorded using, at least in part, an electrode inthe electrode array, that may be used as a negative electrode, and aelectrode positioned proximate the scalp and/or forehead of the body,that may be used as a positive electrode.

In accordance with related embodiments of the invention, the auditoryprosthesis may be a brainstem implant or a cochlear implant. Theelectrode positioned proximate the scalp and/or forehead of the body maybe positioned external or internal to the body. The electrode positionedproximate the scalp and/or forehead of the body may be placedsubstantially proximate the vertex of the head of the body. Theelectrode positioned proximate the scalp and/or forehead of the body maybe operatively coupled to the auditory prosthesis. The auditoryprosthesis may include a connector, the method further comprisingconnecting the electrode positioned proximate the scalp and/or foreheadof the body to the auditory prosthesis via the connector. The connectormay be positioned internal to the body when implanting the auditoryprosthesis, and in connecting the electrode positioned proximate thescalp and/or forehead of the body to the auditory prosthesis via theconnector, the electrode positioned proximate the scalp and/or foreheadof the body is positioned either internally or externally to the body.The at least one electrode in the electrode array may be used forrecording only, or for both recording, and stimulation (i.e., acting asan auditory prosthesis). Stimulating at least one electrode in theelectrode array may include stimulating a plurality of electrodes in theelectrode array.

In accordance with further related embodiments of the invention, themethod may include canceling artifacts from the responses. Cancelingartifacts from the responses may include improved forward maskingcancellation.

In accordance with another embodiment of the invention, a system formeasuring electrically evoked auditory brainstem responses of a patientor animal body includes an auditory prosthesis having an electrodearray, the electrode array for positioning either intracochlear orsubstantially proximate a brainstem of the body. An electrode, which maybe used as a positive electrode, is operatively coupled to the auditoryprosthesis for positioning proximate the scalp and/or forehead of thebody. A controller stimulates at least one electrode in the electrodearray, and records electrically evoked auditory brainstem responsesresulting from said stimulation using, at least in part, an electrode inthe electrode array, which may be used as a negative electrode, and theelectrode for positioning proximate the scalp and/or forehead of thebody.

In accordance with related embodiments of the invention, the auditoryprosthesis may be a brainstem implant or a cochlear implant. Theauditory prosthesis may include a connector for connecting to theelectrode for positioning proximate the scalp and/or forehead of thebody. The connector may removably couple the electrode for positioningproximate the scalp and/or forehead of the body to the auditoryprosthesis. The connector may include means for positioning theconnector internal to the body when implanting the auditory prosthesis.The controller may simultaneously stimulate at least two electrodes inthe electrode array when measuring electrically evoked auditorybrainstem responses.

In accordance with further related embodiments of the invention, thecontroller may cancel artifacts in the responses. The controller maycancel artifacts using improved forward masking cancellation.

In accordance with another embodiment of the invention, a method ofmeasuring electrically evoked auditory brainstem responses of a patientor animal body is provided. The method includes surgically implanting anauditory prosthesis having an electrode array, the electrode arraypositioned one of intracochlear and substantially proximate a brainstemof the body. At least one electrode is stimulated in the electrodearray. Electrically evoked auditory brainstem responses resulting fromsaid stimulation are recorded. Artifacts in the responses are cancelledusing improved forward masking methodology.

In accordance with related embodiments of the invention, recording theelectrically evoked auditory brainstem responses may include using, atleast in part, a positive electrode placed proximate the scalp and/orforehead of the body, and a negative electrode placed substantiallyproximate one of a mastoid, earlobe and preauricular area of an ear. Thepositive electrode and the negative electrode may be placed external tothe body, and may be, for example, surface electrodes and/or needleelectrodes. A ground electrode may be used, and placed, for example, onthe lower forehead.

In accordance with further related embodiments of the invention,recording the electrically evoked auditory brainstem responses mayinclude using, at least in part, a positive electrode placed proximatethe scalp and/or forehead of the body, and an electrode in the electrodearray that acts as a negative electrode. The positive electrode may beoperatively coupled to the auditory prosthesis via a connector. Thepositive electrode may be placed internal or external the body.Stimulating at least one electrode in the electrode array may includestimulating a plurality of electrodes in the electrode array.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1(a) is a block diagram of a conventional system for measuringeABR;

FIG. 1(b) shows typical recording electrode placement for eABR;

FIG. 2 shows an eABR measuring system, in accordance with an embodimentof the invention;

FIG. 3(a-c) shows exemplary ABR and eABR waveforms.

FIG. 4 shows an improved forward masking scheme for applying to an eABRmeasurement, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In illustrative embodiments of the invention, a system and method ofmeasuring electrically evoked auditory brainstem responses (eABR) of apatient or animal body is provided. Generally, electrically evokedauditory brainstem responses are recorded using one or more electrodesin a cochlear or auditory brainstem implant's electrode array (e.g., asa negative electrode), and an electrode positioned proximate the scalpor forehead of the patient (e.g., as a positive electrode). Such aconfiguration advantageously increases the amplitude of the recordedeABR. In various embodiments, artifacts present in the eABR areadvantageously cancelled using a forward masking methodology, and/ormeasurement of the eABR may be performed using a prosthesis withoutrequiring use of a separate recording system. Details are discussedbelow.

FIG. 1(a) is a block diagram of a system for measuring eABR of a patientor animal body. The system includes a control module 101 having, inpart, a stimulation module 105 and a recording module 107. Thestimulation module 105 provides a stimulation input 109 to theprosthesis (for example, a cochlear implant or an ABI), and communicatestriggering information to the recording module 107. Communicationbetween the external and internal portions of the system may be, withoutlimitation, via a transcutaneous radio frequency link, typicallyinvolving an external coil 111 that interfaces with an implant'sinternal coil. Recording electrodes 113 pick up the eABR, which are thenprocessed in the recording module 107. Conventional recording electrodepositions for eABR, shown in FIG. 1(b) may include, without limitation,an active positive electrode 120 placed on the vertex (Cz) and/orforehead, and/or an active negative electrode 121 placed on ipsilateralimplanted—or contralateral unimplanted mastoid, earlobe or preauriculararea of the ear. A reference/ground electrode 122 may be placed eitheron the neck, lower forehead or shoulder (see, for example, Katz et al.,1994).

In illustrative embodiments of the invention, measurement of eABR isperformed via the implanted prosthesis, without requiring a separateexternal recording module. FIG. 2 shows an eABR measuring system, inaccordance with an embodiment of the invention. The system includes animplant 201, which may be, without limitation, a cochlear implant or anABI. The implant 201 includes an electrode array 209 for positioning,without limitation, intracochlear (so as to stimulate the auditorynerve) or substantially proximate a brainstem of the body.

In addition to the electrode array, various recording electrode(s) 211are connected to the implant 201. Cable/wiring 213, as known in the art,may be used to connect the recording electrode(s) 211 to the implant201. The cable/wiring may be, without limitation, flexible to allow foradjustment in positioning the electrode 211.

The recording electrode(s) 211 may be removably and/or irremovablyconnected to the implant 201. In various embodiments, one or morerecording electrode(s) 211 may be fully implantable/fixed in positionand irremovably connected to the implant 201. In further embodiments,the implant 201 may include a connector(s) 205 that is used tointraoperatively couple one or more recording electrodes 211 to theimplant 201. In various embodiments, the plug is positioned internal thepatient. After, for example, the intraoperative measurement, theelectrode(s) 211 (e.g., the vertex electrode) may then be disconnected,and the implant along with the connector 205 may stay inside the body.Recording electrode(s) 211 may be surface or needle electrodes which areintraoperatively placed (on or just under the skin) and connected to theimplant 201 via connector 205. The electrode(s) 211 may then be removedafter the evoked potentials measurement. In alternative embodiments, theconnector may be positioned external the patient.

In various embodiments of the invention, an electrode 211, for example,and without limitation, a positive electrode, may be positioned on orjust under the scalp or forehead of the patient. For example, theelectrode 211 may be positioned at the vertex/top of the patient's head.Additionally, one of the electrodes in the electrode array 209 of theimplant, positioned intracochlear (for a cochlear implant), orsubstantially proximate the auditory brainstem (for an ABI), may beused, without limitation, as a negative electrode. This configurationadvantageously (and surprisingly) provides more robust eABR measurementscompared to previous configurations. The negative electrode 209 may bean already existing electrode in the implant electrode array, such thatit is used for stimulation during normal use of the implant, and forrecording when measuring eABR. Alternatively, the negative electrode 209may not be used for stimulation, and hence may need to be added to theexisting electrode array configuration of the implant. In accordancewith other embodiments of the invention, the implant may attach to amore conventional recording electrode configuration, for example, withthe negative electrode positioned for example near the mastoid as shownin FIG. 1(a).

When conducting eABR measurements, a single electrode channel may beactivated. Alternatively, a plurality of electrode channels may beactivated simultaneously, in accordance with various embodiments of theinvention. For example, a plurality of electrodes within the electrodearray of a cochlear implant or ABI may be simultaneously activated.Activating a plurality of electrodes simultaneously during eABRadvantageously requires lower currents per electrode channel to evoke aresponse compared to one channel stimulation. Thus, eABR measurementsusing simultaneous stimulation can be helpful in subjects with highercurrents needed for eliciting an auditory perception. Additionally,simultaneous stimulation during eABR allows for a generally fasterfitting procedure (i.e., a decreased fitting time) than measuring ofevoked responses with one channel stimulation. The fitting map may alsobe more precise than the approximated fitting map created from the samenumber of recordings with just one channel stimulation. Indifficult-to-fit patients, using multi-channel stimulation responses isgenerally more robust and easier to recognize than responses when usingone channel stimulation.

Simultaneous stimulation of electrodes may be advantageously applied, inaddition to or in combination with the above-described eABR embodiments,to all evoked potential methodologies useful for fitting of implantprostheses. Such methodologies may include, without limitation: earlylatency evoked auditory responses (e.g., eABR); middle latency evokedauditory responses (e.g., MLR, 40 Hz) or late evoked auditory response(e.g., P300, P1N1P2) and muscle reflex responses (e.g., PAMR).

An exemplary methodology/implementation of multi-channel stimulation ofevoked potentials/muscle reflexes for fitting an implant is nowdescribed. The methodology may be applied to any evoked potentialprocedure used for the fitting of an implant prosthesis.

1. Select number of channels n being stimulated at the same time. Theselected electrodes should be in order; for example, if n=3, electrodes1-3 are stimulated, followed by electrodes 4-6, electrodes 7-9,electrodes 10-12 . . . .

2. Measure the eABR response (the stimulating level is increased until aclear response is seen).

3. Decrease the stimulating level and determine the stimulation level atwhich the objective response disappears. This stimulation level at whichthe objective response disappears is called the Threshold Reference(TR). For eABR, Wave 5 is usually observed as it is most robust. FIG.3(a-c) shows exemplary ABR and eABR waveforms. More particularly, FIG.3(a) shows an exemplary acoustically evoked ABR; FIG. 3(b) shows anexemplary electrically evoked ABR for a cochlear implant subject; andFIG. 3(c) shows an example of an electrically evoked ABR for an ABIsubject.

4. Determine mean Most Comfortable Level (MCL) for the selected group ofelectrodes. The mean MCL for the selected group of electrodes can becalculated, based on, without limitation, the equation TR/0.6 n.

5. Determine the mean Threshold (T) for the selected group ofelectrodes. The mean T can be calculated, without limitation, basedapproximately on the equation TR/2.4 n (25% of the MCL, or four timeslower than the MCL).

6. Select the next group of electrodes (electrodes (n+1, . . . , 2 n)and repeat steps 2-5. For example, if n=3, electrodes 4, 5 and 6 areselected.

7. When all the electrodes have been selected, stop measurements.

8. Determine approximate MCL for each single electrode. Using a linearapproximation, the approximate MCL can be calculated based on, withoutlimitation, the following equations:

  n = 4  (even)E 1MCL = MCLGroup 1 + (n/2) * (MCLGroup 1 − MCLGroup 2)/nE 2MCL = MCLGroup 1 + (n/2 − 1) * (MCLGroup 1 − MCLGroup 2)/nE 3MCL = MCLGroup 1 − (n/2 − 1) * (MCLGroup 1 − MCLGroup 2)/nE 4MCL = MCLGroup 1 − (n/2) * (MCLGroup 1 − MCLGroup 2)/nE 5MCL = MCLGroup 2 + (n/2) * (MCLGroup 1 − MCLGroup 2)/nE 6MCL = MCLGroup 2 + (n/2 − 1) * (MCLGroup 1 − MCLGroup 2)/nE 7MCL = MCLGroup 1 − (n/2 − 1) * (MCLGroup 2 − MCLGroup 3)/nE 8MCL = MCLGroup 1 − (n/2) * (MCLGroup 2 − MCLGroup 3)/n   ⋮  n = 3  (odd)   E 1MCL = MCLGroup 1 + (MCLGroup 1 − MCLGroup 2)/3  E 2MCL = MCLGroup 1  E 3MCL = MCLGroup 1 − (MCLGroup 1 − MCLGroup 2)/3  E 4MCL = MCLGroup 2 + (MCLGroup 1 − MCLGroup 2)/3  E 5MCL = MCLGroup 2  E 6MCL = MCLGroup 2 − (MCLGroup 2 − MCLGroup 3)/3   ⋮

Determine approximate Threshold T for each single electrode. Using alinear approximation, the approximate T can be calculated based on,without limitation, the following equations:

n = 4  (even) E 1T = TGroup 1 + (n/2) * (TGroup 1 − TGroup 2)/nE 2T = TGroup 1 + (n/2 − 1) * (TGroup 1 − TGroup 2)/nE 3T = TGroup 1 − (n/2 − 1) * (TGroup 1 − TGroup 2)/nE 4T = TGroup 1 − (n/2) * (TGroup 1 − TGroup 2)/nE 5T = TGroup 2 + (n/2) * (TGroup 1 − TGroup 2)/nE 6T = TGroup 2 + (n/2 − 1) * (TGroup 1 − TGroup 2)/nE 7 T = TGroup 1 − (n/2 − 1) * (TGroup 2 − TGroup 3)/nE 8 T = TGroup 1 − (n/2) * (T Group 2 − T Group 3)/n⋮n = 3  (odd)E 1T = TGroup 1 + (TGroup 1 − TGroup 2)/3E 2T = TGroup 1E 3T = TGroup 1 − (TGroup 1 − TGroup 2)/3E 4T = TGroup 2 + (TGroup 1 − TGroup 2)/3E 5T = TGroup 2E 6T = TGroup 2 − (TGroup 2 − TGroup 3)/3⋮

In various embodiments, T levels may not be calculated, or T for eachelectrode may be obtained, for example, based on T=0.1 (MCL). Theabove-described equations assume that the electrodes being stimulatedare properly inserted in the cochlea. If any of the electrodes ismalfunctioning or extra-cochlear, these electrodes should be excludedfrom the stimulation, and the MCL and T calculations adjustedaccordingly. The above-described methodology is applicable to eCAP, inwhich case TR is defined as the maximum stimulating level at which noCAP response is elicitable. The calculations of MCL and T may need to beappropriately adjusted to account for various types of simulationproperties of an implant prosthesis. In various embodiments, theconstant value of 0.6 and 2.4 in the calculation of the MCL group(TR/0.6n) and T group (TR/2.4n) may be different for different electrodearrays.

In further illustrative embodiments of the invention, artifactcancellation by subtraction methodologies is applied to eABRmeasurements. In preferred embodiments, the so called “improved forwardmasking method” may be utilized, similar to that used for the recordingof CAP responses, but never before applied to eABR measurements (seeMiller C. A. et al., An improved method of reducing stimulus artifact inthe electrically evoked whole-nerve potential, Ear Hear. 2000 August;21(4), p. 280-90 at FIG. 2, which is hereby incorporated herein byreference). eCAP recordings are sensitive to stimuli artifacts,electrode or implant artifacts and miogenic type artifacts. In additionto the above mentioned artifacts for eCAP, eABR is very sensitive toartifacts caused by patients (i.e. patient's movement) and theirsurroundings. The above-described embodiments allow for more stableelectrode placement (in particular, embodiments in which the recordingelectrodes are implanted or otherwise fixed in position), leading tomore favourable cancellation using subtraction methodology.

FIG. 4 shows an “improved forward masking” scheme for applying to aneABR measurement, in accordance with an embodiment of the invention. Itis to be understood that use of other mathematical artifact cancellationmethodologies, as known in the art, such as the subthreshold templatemethodology or standard forward masking methodology may also beutilized. Illustratively, with reference to FIG. 4, a probe-alonecondition is not recorded (thus, the assumption in standard forwardmasking methodology that the morphology of the partially masked EAP isidentical to that of the unmasked EAP is no longer needed). Instead,direct measurement of the EAP to a masked probe at various masker-probeintervals are taken. In FIG. 4, trace B shows a stimulus that elicits apartially refractory response. As shown in Trace E, the probe artifactin B is subtracted by recording the forward-masked probe using amasker-probe interval having absolute refractoriness (E). To eliminatethe masker stimuli introduced in B and E, recordings of the maskerresponse C and a time-shifted masker response C′ are utilized. Theresultant waveform (R₂), equal to B−E−C+C′ has eliminated the probestimulus artifact.

Advantages of using the above-described implant recording system withelectrodes for CI and ABI subjects include: improved artifactcancellation; less sweeps for each recording will result in decreasedclinical time/faster recordings; the recording electrode configurationof eABR recordings described in the above embodiments will decreaseartifacts caused by patient (i.e. patients movement), particularly wherefixed electrodes are used; less sensitivity to possible errors,particularly where stimulating and recording electrodes have fixedpositions—subjects will have more freedom to move during themeasurement; and it is easier to perform (no need of an external systemfor recording, no need to connect both recording system and system forstimulation, and no need to place the recording electrodes (inembodiments where electrodes are fixed in position).

It is to be understood that the above-described embodiments may be usedfor other types of electrically evoked potentials (EEP) measurements(e.g., including early, middle and late latency auditory responses),however, the placement and number of the recording electrodes may differin dependency on what type of EEPs is being measured. For example, whenmeasuring late latency potentials, additional Cz electrodes may beadded.

Embodiments of the invention may be implemented in whole or in part as acomputer program product for use with a computer system. Suchimplementation may include a series of computer instructions fixedeither on a tangible medium, such as a computer readable medium (e.g., adiskette, CD-ROM, ROM, or fixed disk) or transmittable to a computersystem, via a modem or other interface device, such as a communicationsadapter connected to a network over a medium. The medium may be either atangible medium (e.g., optical or analog communications lines) or amedium implemented with wireless techniques (e.g., microwave, infraredor other transmission techniques). The series of computer instructionsembodies all or part of the functionality previously described hereinwith respect to the system. Those skilled in the art should appreciatethat such computer instructions can be written in a number ofprogramming languages for use with many computer architectures oroperating systems. Furthermore, such instructions may be stored in anymemory device, such as semiconductor, magnetic, optical or other memorydevices, and may be transmitted using any communications technology,such as optical, infrared, microwave, or other transmissiontechnologies. It is expected that such a computer program product may bedistributed as a removable medium with accompanying printed orelectronic documentation (e.g., shrink wrapped software), preloaded witha computer system (e.g., on system ROM or fixed disk), or distributedfrom a server or electronic bulletin board over the network (e.g., theInternet or World Wide Web). Of course, some embodiments of theinvention may be implemented as a combination of both software (e.g., acomputer program product) and hardware. Still other embodiments of theinvention are implemented as entirely hardware, or entirely software(e.g., a computer program product).

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention.

What is claimed is:
 1. A method of fitting an auditory prosthesis usingelectrically evoked auditory brainstem responses of a patient or animalbody, the prosthesis including an electrode array, the electrode arraydivided into a plurality of predefined groups, the method comprising:simultaneously stimulating the predefined group of electrodes at astimulation level; for each predefined group of electrodes, increasingthe stimulation level until a measured evoked auditory brainstemresponse is observed using an electrode positioned proximate the scalpand/or forehead of the body; decreasing the stimulation level todetermine the Threshold Reference (TR) stimulation level at which themeasured evoked auditory brainstem response disappears; and determining,based at least in part on Threshold Reference (TR), at least oneparameter selected from the group consisting of a mean Most ComfortableLevel (MCL) and a mean Threshold (T) for the predefined group ofelectrodes; determining at least one parameter selected from the groupconsisting of Most Comfortable Level (MCL) and Threshold (T) for eachelectrode in the electrode array; and fitting the auditory prosthesiswith the at least one parameter selected from the group consisting ofMost Comfortable Level (MCL) and Threshold (T) for at least oneelectrode in the electrode array.
 2. The method according to claim 1,wherein the auditory prosthesis is one of a cochlear implant and abrainstem implant.
 3. The method according to claim 2, whereinincreasing the stimulation level until a measured evoked auditorybrainstem response is observed includes using an electrode positionedproximate the scalp and/or forehead of the body.
 4. The method accordingto claim 3, wherein the electrode positioned proximate the scalp and/orforehead of the body includes placing the electrode substantiallyproximate the vertex of the head of the body.
 5. The method according toclaim 3, wherein the electrode positioned proximate the scalp and/orforehead of the body is connected to the auditory prosthesis via wire.6. The method according to claim 1, wherein the electrodes in theelectrode array are sequentially ordered relative to their position withthe electrode array, and wherein the electrodes within the predefinedgroups of electrodes are in a sequential order.
 7. The method accordingto claim 1, wherein each predefined group of electrodes includes thesame number of electrodes, and wherein each electrode is associated withonly one group of electrodes.
 8. The method according to claim 1,wherein each predefined group of electrodes has N number of electrodes,and wherein determining the mean Most Comfortable Level (MCL) is basedon the equation: mean MCL=TR/((a constant value)N).
 9. The methodaccording to claim 8, where the constant value is 0.6.
 10. The methodaccording to claim 1, wherein each predefined group of electrodes has Nnumber of electrodes, and wherein determining the mean Threshold (T) isbased on the equation: T=TR/((a constant value)N).
 11. The methodaccording to claim 10, wherein the constant value is 2.4.
 12. The methodaccording to claim 1, wherein each predefined group of electrodes has Nnumber of electrodes, and wherein determining the mean Threshold (T) isbased on the equation: T=0.1MCL.
 13. The method according to claim 1,wherein determining at least one parameter selected from the group ofparameters consisting of Most Comfortable Level (MCL) and Threshold (T)for at least one electrode in the electrode array includes using alinear approximation.